The present invention relates to a method for producing a substrate tube of quartz glass, comprising continuously supplying to a heating zone a hollow cylinder of quartz glass having an outer diameter Ca, an inner diameter Ci and an inner bore, zone-wise softening the hollow cylinder in the heating zone, and drawing off from the softened portion a tubular strand with an outer diameter Ta and an inner diameter Ti.
Substrate tubes of the desired length are obtained by cutting the tubular strand to length. They serve as semi-finished product in the manufacture of preforms for optical fibers. Additional glass layers are here deposited from the gas phase on the inner wall of the substrate tube.
A generic vertical drawing method for producing a substrate tube is known from WO 2004/083141 A. A hollow cylinder of synthetic quartz glass is supplied from above in a vertical orientation of the longitudinal axis of the cylinder to a furnace having an annular heating element of graphite. The hollow cylinder has an outer diameter Ca of 150 mm and an inner diameter Ci of 70 mm. The diameter ratio Cr=Ca/Ci is thus about 2.14. After the furnace has been heated to its target temperature of about 2,300° C., the hollow cylinder is continuously moved into the furnace at a lowering rate of 11 mm/min and thereby softened zone by zone. At the same time a tubular strand with an inner diameter Ti of 22 mm and an outer diameter Ta of 28 mm is drawn off by means of a draw-off device at a rate of 640 mm/min. The diameter ratio in the tubular strand Tr=Ta/Ti is thus about 1.27.
During the drawing process, the outer diameter and the wall thickness of the drawn-off tubular strand are kept constant by way of a process control. The internal pressure within the inner bore of the hollow cylinder serves as a control variable. The pressure is produced and maintained by introducing a nitrogen stream into the inner bore of the hollow cylinder. The nitrogen used is first dried to prevent the incorporation of hydroxyl groups (OH groups) into the quartz glass of the inner wall. The nitrogen flow rate (about 30 L/min) is configured such that a blow pressure of about 1.5 mbar is obtained. The lower end of the drawn-off tubular strand is closed, in part, by means of a plug to avoid an unhindered outflow of the stream of nitrogen and a resulting cooling of the inner wall of the drawn-off quartz glass tube by the gas flow. A smooth inner wall is obtained that is distinguished by a mean surface roughness Ra of 0.06 μm.
The glass tube produced thereby is cut to suitable segments and is used as a substrate tube for the deposition of SiO2 layers on the inner wall by means of an MCVD method.
The inner wall of the substrate tube forms a contact surface with another, subsequently added glass which either belongs to the core of the optical fiber or is adjacent to the fiber core. Therefore, the inner wall of the substrate tube must, in principle, be free of flaws and impurities.
The drying of the nitrogen stream as suggested in WO 2004/083141 A1 represents an effective measure for limiting the incorporation of hydroxyl groups into the quartz glass of the inner wall, but makes the substrate tube production, which is already cost-intensive, even more expensive.
It has also been suggested that the inner wall of the substrate tube should be removed in the end, for instance by mechanical milling or by etching, to eliminate impurities contained on or in the surface layer. These procedures are, however, complicated and slow, and they form additional sources for impurities and surface defects.
The known elongation methods for producing substrate tubes are typically vertical drawing methods without any tools. A molding tool is not used in the formation of the outer wall or in the formation of the inner wall. The reason is that vapors and particles released from a molding tool or drawing streaks formed by mechanical contact could otherwise ruin the outer cylinder surfaces of the substrate tube.
However, the shaping of the initial hollow cylinder without tools entails problems regarding compliance with the nominal radial dimensions of the tube strand and the rotation symmetry. Specifically, high-frequency diameter variations as well as ovality in the radial cross-sectional profile or wall one-sidedness are noticed, i.e. radially irregular profiles of the tube wall thickness, which is also called “siding” among the experts. These problems are the more pronounced, the stronger the shaping process from the hollow cylinder into the tube. A measure thereof is the so-called “elongation ratio” or “draw ratio”. This measure designates the ratio of the lengths of drawn-off tubular strand and start cylinder.
To improve the dimensional stability in elongation processes having a high draw ratio, many measures have been suggested, e.g., rotating the start cylinder about its longitudinal cylinder axis, the use of short or long heating zones or optimizing the gap width between the heating zone and the hollow cylinder. However, a reproducible transfer of these measures to the production of substrate tubes by way of elongation without the use of tools turns out to be difficult.
It is therefore an objective of the present invention to provide an elongation process for the cost-effective production of substrate tubes of high dimensional stability and surface quality.